Certain advanced radar systems under development require noise and spurious performance well beyond that of currently fielded systems. The noise and spurious performance of the variable frequency oscillator (VFO) used for Doppler tuning of main lobe clutter must, in particular, be dramatically improved to be compatible with these application needs. While that improvement may be feasible, the simultaneous improvements in noise, spurious signal content and switching speed is viewed as a design risk. Phase tuning rather than frequency tuning can also be used to effect the Doppler tuning.
The phase tuning approach requires a precision phase shifter. The prior art for electrically tuned phase shifters is wide and diverse, including analog designs using varactor tuning, switched line lengths, tapped delay lines, vector modulators, n-stage reactively terminated quadrature hybrids, and direct digital synthesizers (DDS). With the exception of the DDS, the phase resolution and accuracy that can be achieved from these approaches is limited by fabrication precision, alignment, and parasitic effects. The DDS can achieve the phase resolution of the disclosed approach but, without specialized and complex added circuitry, does not provide the spurious signal performance required for radar application.